Attempts have been made to maximize reception sensitivity of an optical receiver using an avalanche photo diode (APD) in a transmission system. APDs that sufficiently secure a required APD gain and bandwidth are used as the APD in the transmission system. Therefore, it was unnecessary to worry about the bandwidth of the APD when the optical receiver is implemented. Accordingly, existing inventions focus on configurations for maximizing characteristics of the APD.
For example, techniques include configurations for APD bias control, such as APD bias control for controlling gain of the APD and for improving the signal-to-noise (S/N) ratio of a received signal. Other techniques include compensation circuit for forming an optimal APD bias according to temperature, employing a method of controlling the APD bias by storing an optimal APD voltage in a read-only memory (ROM), searching for an optimal APD bias using a thermistor, and an APD bias circuit for preventing an APD from being damaged when a large amount of light is input to the APD because gain is obtained in photoelectric conversion. Still other techniques include calibration of a photoelectric element using the breakdown voltage of the APD.
As a transmission rate of optical transmission systems is increased to 10 Gb/s or more, and use of smart devices is rapidly increasing, the transmission capacity or transmission rate of optical transmission networks has recently been increased in order to catch up with the rate of traffic increase. In a light communication system, an optical receiver is necessarily used to receive an optical signal transmitted. In a case where it is necessary to transmit light over a long distance or in a case where a large light link margin is required, the light receiver uses an APD that improves its characteristics by amplifying a photoelectric converted signal. In the optical receiver, an APD having a sufficient bandwidth suitable for a required transmission rate is used, thereby maximizing characteristics of the APD. It is difficult to catch up with the recent increase in transmission rate and to manufacture a high-speed APD. It is desirable to have an optical transmission system that overcomes such difficulties.